Desulfurization of hot fuel gas could lead to thermally efficient power generation in coal-based, combined-cycle systems and also has advantages for coal-based direct reduction of iron ore. Many hot-desulfurization processes use calcium oxide as the desulfurizing agent (cf. U.S. Pat. Nos. 3,276,203; 3,307,350 and 3,853,538). Dolomite is a preferred form of calcium oxide. It forms calcium sulfide by reaction with the hydrogen sulfide in the coal-derived gas. While calcium oxide is an effective gas-desulfurizing agent, the most commonly proposed method of regenerating calcium sulfide, reacting with CO.sub.2 and H.sub.2 O under slightly reducing conditions at pressures greater than about 50 psig and temperatures preferably about 1000.degree. to 1200.degree. F., and calcining the calcium carbonate formed in the regeneration to return it to the calcium oxide form, does not achieve complete removal of the sulfur from the material. Thus, continuous use of calcium oxide for gas desulfurization followed by regeneration results in successively less efficient regeneration until, after about 10 cycles, about 10 to 20 percent of the original desulfurizing capacity remains. Furthermore, the spent material must undergo expensive and complex treatment to bring it to a state suitable for disposal to avoid pollution of the air and ground water.
In our process, the regenerated calcium oxide has improved capacity for reaction with sulfur-bearing fuel gas and repeated regeneration of the sulfided calcium oxide by this process does not induce significant progressive loss of capacity. The regenerated calcium oxide can retain approximately 70 to 80 percent of its original sulfur-holding capacity, which makes it suitable for repeated cyclic use. U.S. Pat. No. 4,212,851, incorporated herein by reference, for a general description of this improved process. The preferred regeneration temperature range of 1800.degree. to 2050.degree. F. will cause some shrinkage and decrease in surface area, but the cycle time can be adjusted to compensate for any concomitant decrease in reactivity. When it is necessary to dispose of the calcium oxide (for example, because of an excessive accumulation of fly ash in the bed or because of excessive loss of reactivity or capacity), any residual sulfur after the completion of the final regeneration period will be in the form of calcium sulfate and pose no air or water pollution problems. This improved process operates at temperatures commonly used to desulfurize gases, such as fuel gas, so that no temperature swing between the gas-desulfurization step and the calcium oxide-regeneration step is necessary, thus avoiding delays and complexities in the sequencing of process steps.
One of the problems that may arise in any of the recycle calcium oxide processes is that undesirable side reactions occur which result in loss of calcium oxide desulfurization capacity. Thus, sulfating of the calcium oxide can occur in the upper portion of a fixed bed of calcium oxide if the upper portion of the bed is left unsaturated after the gas desulfurization step. This sulfate formation occurs because the remaining calcium oxide is available to react with the sulfur dioxide generated in the lower portion of the bed and residual oxygen in the regeneration gas. Also, if the upper portion of the bed is not completely regenerated, the coal-gasification gas passing through the bed during the next gas desulfurization period containing hydrogen, hydrogen sulfide, carbon monoxide, carbon dioxide, and water vapor will deposit the incoming sulfur in the completely regenerated lower portion of the bed and will pick up sulfur in the upper incompletely regenerated portion of the bed. Thus, the fuel gas will not be sufficiently low in hydrogen sulfide at the outset of the gas-desulfurization period.
Attempts to prevent this problem by making sure that the bed is completely sulfided in the desulfurization step allows undesirable hydrogen sulfide to remain in the gas stream because of insufficient contact time with the small amount of calcium oxide remaining in the bed.